The Bigeye bomb, centerpiece of President Reagan's $174 million plan to modernize the U.S. chemical-weapons arsenal, remains technically flawed despite seven years of testing, according to the General Accounting Office.

In preliminary findings presented to House Foreign Affairs Committee Chairman Dante B. Fascell (D-Fla.), the GAO reported that in eight of its last nine tests by the Army, the bomb had not produced a sufficiently lethal chemical reaction at high temperatures.

As the House prepares to vote next month on Reagan's proposal to end a 16-year moratorium on production of chemical weapons, the findings are expected to strengthen opposition to Bigeye.

The administration, however, is expected to argue that the bomb achieved adequate toxicity levels at all temperatures.

The bomb is called a binary weapon because its lethal punch comes from mixing two nontoxic chemicals that produce nerve gas after Bigeye is dropped.

The Senate approved Bigeye production last week, raising administration hopes for resumption of the chemical-weapons program after three years of rejection by Congress. The proposal has also won approval of the House Armed Services Committee but is expected to encounter strong opposition in the full House, where it was soundly defeated last year.

Fascell, a leading House opponent of Bigeye, had asked the GAO for a technical evaluation of the bomb and received initial findings in an oral report May 9.

In a subsequent letter to the comptroller general's office, he noted that "technical deficiencies of the binary-weapons program persist" and asked for a detailed examination.

The GAO told Fascell that Bigeye had not met specifications for a certain level of chemical toxicity at temperatures above 120 degrees. At high temperatures, the GAO said, the bomb explodes, leaks or falls short of the "chemical purity" required to achieve sufficient lethality.

Bigeye, which the administration says will strengthen the deterrent against the Soviets' larger chemical arsenal, has been plagued by technical problems since testing began in 1978. Three years ago, the Army discovered that the bomb might explode on its own and spew deadly gas before being dropped from an aircraft.

Administration officials have told Congress in recent weeks that the technical problems have been resolved, and Deputy Assistant Defense Secretary Thomas J. Welch said in an interview that Bigeye is lethal enough to kill enemy troops.

"It's a bonus, and we're not going to give it away because the chemical isn't pure," he said.

Reagan's bid for $174 million to produce Bigeye and 155mm artillery shells is designed to upgrade an aging stockpile of nerve gas, less than 10 percent of which is said to be militarily usable. Congress has refused to finance new chemical weapons since 1969, while allowing funds for research and development and defensive measures against nerve gas.

Another $936 million is being sought for next year for protective gear and training against chemical attack.

In a separate report in April, the GAO found serious deficiencies in military defensive measures against such an assault, despite nearly $2 billion in congressional appropriations in the past three years.

Investigators discovered that soldiers are outfitted with flammable boots and gloves and with masks that do not protect against high concentrations of carbon monoxide. They lack skin decontaminants and a medical antidote effective against the kind of nerve gas believed to have been developed by the Soviets.

The report cited an acute shortage of medics and troops trained for gas warfare, with the Army fielding only one-third of the 24,000 such personnel it plans to have by fiscal 1987.

The GAO also said that many commanders ignore requirements to have troops train in protective gear and some do not include medical participation in the exercises or practicing such functions as eating and eliminating body waste while wearing the gear.